* Human fish supplies from oceans at risk towards 2050-study
* Average maximum weights for fish to fall by 14-24 percent
By Alister Doyle
OSLO, Sept 30 (Reuters) - Fish are likely to get smaller on average by 2050 because global warming will cut the amount of oxygen in the oceans in a shift that may also mean dwindling catches, according to a study on Sunday.
Average maximum body weights for 600 types of marine fish, such as cod, plaice, halibut and flounder, would contract by 14-24 percent by 2050 from 2000 under a scenario of a quick rise in greenhouse gas emissions, it said.
"The reductions in body size will affect whole ecosystems," lead author William Cheung of the University of British Columbia in Canada, told Reuters of the findings in the journal Nature Climate Change.
His team of scientists said a trend towards smaller sizes was "expected to have large implications" for ocean food webs and for human "fisheries and global protein supply."
"The consequences of failing to curtail greenhouse gas emissions on marine ecosystems are likely to be larger than previously indicated," the U.S. and Canada-based scientists wrote.
They said global warming, blamed on human burning of fossil fuels, will make life harder for fish in the oceans largely because warmer water can hold less dissolved oxygen, vital for respiration and growth.
"As the fish grow bigger and bigger it will be difficult to get enough oxygen for growth. There is more demand for oxygen as the body grows. At some point the fish will stop growing," Cheung said of the study, based on computer models.
As water gets warmer, it also gets lighter, limiting the mixing of oxygen from the surface layers towards the colder, denser layers where many fish live. Rising water temperatures would also add stresses to the metabolic rates of fish.
The scientists said fish stocks were likely to shift from the tropics towards cooler seas to the north and south.
Average maximum sizes of fish in the Indian Ocean were likely to shrink most, by 24 percent, followed by a decline of 20 percent in the Atlantic and 14 percent in the Pacific. The Indian Ocean has most tropical waters of the three.
The study said a computer model projected that ranges for most fish populations would shift towards the poles at a median rate of 27.5 km to 36.4 km (17.1-22.6 miles) a decade from 2000 to 2050.
Adding to climate change, other human factors "such as over-fishing and pollution, are likely to further exacerbate such impacts," they wrote.
Cheung said the shrinking of fish would have big but unknown effects on marine food chains. Predator fish like cod that swallow prey whole would become less fearsome, perhaps allowing smaller species to thrive.
"Cod ... can only eat fish that can fit into their mouth. They are not like lions or tigers" that can attack animals that are larger than they are, he said.
The climate scenario used in the study would mean an increase in world temperatures of between 2 and 5.4 degrees Celsius (3.6 to 9.7 Fahrenheit) by 2100, the second biggest gain of six scenarios used by the U.N. panel of climate experts.
"The results will be quite similar," using other scenarios, Cheung said. (Reporting By Alister Doyle; Editing by Sophie Hares)